The present application relates to an electro-wetting apparatus using an electro-wetting phenomenon. In addition, the present application relates a varifocal lens, an optical pick-up apparatus, an optical recording/reproducing apparatus, a droplet operating apparatus, an optical device, a zoom lens, an imaging apparatus, a light modulator, a display apparatus, and a strobe apparatus. Furthermore, the present application relates to a method of driving electro-wetting apparatus.
In an optical system of an optical apparatus such as a still camera or a video camera, a mechanism of driving a lens along an optical axis has been adopted to realize focusing operation and zooming operation. However, removal of such a mechanism is considered as effective measures to satisfy a demand of miniaturization of an optical apparatus.
In order to meet the above demands, a varifocal lens using an electro-wetting phenomenon (electric capillary phenomenon) has been proposed (see, for example, Published Japanese translation of a PCT application No. 2001-519539 (JP 2001-519539 A)). In addition, an optical apparatus using such a varifocal lens and a driving method thereof have been also proposed (see, for example, Japanese Unexamined Patent Application Publication No. 2001-249261 (JP 2001-249261 A)).
In the varifocal lens proposed in JP 2001-519539 A, a first liquid with conductivity and a second liquid with insulation properties are hermetically sealed. Here, the second liquid has the same specific gravity as that of the first liquid but may not be mixed therewith. Furthermore, the degree of deformation of the interface profile between the first liquid and the second liquid is controlled by adjusting the amplitude of AC voltage applied to the varifocal lens to control a focus distance. Thus, a mechanism for driving a lens when performing focusing and zooming operation is not necessary even though other lenses demand such a mechanism. In other words, the use of a varifocal lens proposed in JP 2001-519539 A allows a user to perform focusing and zooming operation without a mechanism of driving a lens.
However, the varifocal lenses proposed in JP 2001-519539 A and JP 2001-249261 A can be activated by application of AC voltage with an amplitude of about ±100 V. Thus, it is difficult to directly apply any of these technologies to a small-sized optical apparatus which is based on battery operation. Thus, it has been demanded to drive the varifocal lens using an electro-wetting phenomenon by application of lower voltage.
In order to meet the above request, a technology of using a metal anodized film (dielectric film) as an insulating film for electrically separating an electrode and a conductive liquid has been proposed (see, for example, Japanese Unexamined Patent Application Publication No. 2006-285031 (JP 2006-285031 A)). In JP 2006-285031 A, examples of the insulating film for electrically separating the electrode and the conductive film include a tantalum pentoxide obtained by anodization of metallic tantalum. Therefore, the use of such an insulating film allows a user to control a focus distance within a wide range at an AC voltage of about ±10V.
By the way, in the apparatus using an electro-wetting phenomenon, when a DC voltage is applied between an anode and a cathode, electric charges are accumulated on a dielectric body (insulator) arranged between the anode and the cathode. Thus, the effects of electro-wetting phenomenon acting between the anode and the conductive liquid can be cancelled out. Therefore, when a DC current is applied between the anode and the cathode using the electro-wetting phenomenon, the deformed liquid profile due to the electro-wetting phenomenon is hardly kept. For overcoming such a disadvantage, in any of JP 2001-519539 A, JP 2001-249261 A, and JP 2006-285031 A, an AC voltage is used as one to be applied to the varifocal lens to keep the deformed interface profile of the liquid due to the electro-wetting phenomenon.
However, as is proposed in JP 2006-285031 A, the electric insulating characteristic of the varifocal lens can be represented as shown in FIG. 27 when an anodized metal oxide film is used as an insulating film. In FIG. 27, the horizontal axis represents an applied voltage level and the vertical axis represents a current level passing through the varifocal lens (insulating film). The electrical insulating characteristics shown in the figure are similar to those of general tantalum capacitor, aluminum electrolytic capacitor, or the like. Therefore, if the potential of the anode is higher than that of the cathode, then the insulating film of the varifocal lens can act as a capacitor because positive withstand voltage VEW+ is too large when a positive voltage is applied.
However, the negative withstand voltage VEW− of the insulating film is extremely very small (for example, several volts) compared with the positive withstand voltage VEW− (for example, about several tens to a hundred and several tens of voltages). Thus, when a negative (reverse) voltage is applied to the insulating film, dielectric breakdown of the insulating film can be occurred by the application of only a comparatively small voltage value, resulting in electric conduction. In this case, the varifocal lens does not act as a device using an electro-wetting phenomenon. Therefore, the variable range of the voltage can be restricted by negative withstand voltage VEW−. Thus, there is a problem in that the variable voltage range can be narrowed.
The present invention has been made in consideration of the aforementioned problem and it is desired to provide an electro-wetting apparatus which can be operated at a low voltage within a wider variable voltage range, and various apparatuses provided with such an electro-wetting device, and a driving method thereof.